[Gllug] Missing RAM

[Nix]

On 17 May 2009, Peter Corlett said:

> The figure reported by free is the amount of memory actually available  
> for use. It excludes the space occupied by the kernel, and the space  
> marked off-limit by the BIOS due to it being occupied by legacy  
> hardware.

This space can be *very* large, especially on machines with >4Gb RAM
without an IOMMU. You'll always lose 1/64th of your RAM to page tables
(and if you use memory controllers you lose even more that way), and if
you have >4Gb you'll lose a biggish chunk of RAM below the 4Gb bound to
software bounce buffers ('SWIOTLB') to allow PCI cards to do DMA to
memory they can't otherwise access. (This is one of many reasons why
bandwidth-hungry stuff is often on PCIe on such systems.)
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